This Background will explain why the lack of absolute certainty in lifetimes of igniters used in gas turbine aircraft engines can impose significant costs on the owners of the aircraft utilizing the engines.
FIG. 1 is a highly schematic illustration of a gas turbine engine 3, containing a combustor 6. Fuel 9 is sprayed into the combustor. An igniter 12, which functions in a roughly analogous manner to a spark plug in an automobile, produces a spark, or plasma discharge (not shown), which initially ignites the jet fuel.
After initial ignition, the igniter 12 can be repeatedly sparked thereafter, primarily as a safety measure. That is, in a modern engine, under normal circumstances, it is extremely unlikely for a flame-out to occur in the combustor 6. However, unexpected situations, such as an abrupt cross-wind, can affect the environment within the combustor, and resulting loss of flame.
In addition, certain flight conditions make the unlikely event of a flame-out slightly more probable. Thus, for example, the igniter 12 may be activated when the aircraft enters a rain squall, or other situation which may disturb steady-state conditions in the combustor 6.
The igniters 12, like all mechanical components, have useful lives which eventually expire, at which time the igniters must be replaced. However, this expiration-and-replacement can create a situation in aircraft which is expensive.
A primary reason is that the approach of an igniter to the end of its lifetime is not marked by readily detectable events. That is, at some point, the igniter completely ceases to generate a plasma, or spark. However, prior to that point, the igniter may sporadically generate sparks.
As explained above, the sparking is not, in general, required to maintain the combustor flame. Consequently, the sporadic sparking would only be noticed if an actual flame-out occurred, and if the sporadic sparking were ineffective to induce a re-light. Since such a combination of events is seen as unlikely, the sporadic sparking is not readily noticed. The impending expiration of the useful life of the igniter is similarly not noticed.
Another reason is that, while all igniters may be constructed as identically as possible, nevertheless, those igniters do not all possess the same lifetimes. Nor do all igniters experience identical events during their lifetimes. Thus, it is not known exactly when a given igniter will expire.
Thus, the point in time when an igniter must be replaced is not known with certainty. One approach to solving this problem is to perform preventative maintenance, by replacing the igniters when they are still functioning. While the cost of a new igniter and the manpower required to install it is not great, the early replacement does impose another cost, which can be significant.
The aircraft in which the igniter is being replaced represents a revenue source measured in thousands of dollars per hour. If the aircraft is rendered non-functional for, say, two hours during replacement of an igniter, the revenue lost during that time is substantial.
Therefore, the uncertain lifetimes of igniters in gas turbine aircraft engines can impose significant losses in revenue.